The present application claims priority to Japanese patent application nos. JPAP 2002-328637, filed in the Japanese Patent Office on Nov. 12, 2002, and JPAP 2003-155197, filed in the Japanese Patent Office on May 30, 2003, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a method and apparatus for an ultra wideband communications system employing a spread spectrum technique transmitting a baseband signal over a wide frequency band. Specifically, the present invention relates to a transmitter for ultra wideband communications in which input data is spread by a spread modulator to a wideband signal having a form in which a plurality of carriers are constantly transmitted as baseband signals by wired or wireless transmission, and a method of producing the input data in the transmitter. The present invention also relates to a receiver for ultra wideband communications for reproducing a received signal, and a method of reproducing the received signal to original data in the receiver. The present invention also relates to an ultra wideband communications system including the above-described transmitter and the above-described receiver, and a communications method using the above-described method of producing the input data and the above-described method of reproducing the original data.
2. Description of the Related Art
Conventional communications technologies have effectively utilized limited frequency resources by minimizing use of a frequency bandwidth. That is, the conventional communications technologies have generally used frequency in a narrow bandwidth. However, as the width of a frequency band becomes narrower, tolerance for noise in a communications path and for coherency from signal distortion affected by another signal of another channel are more deteriorated, and thereby, data signals become more susceptible to interference. As a result, the data signals may be unnecessarily transmitted using an extra bandwidth of a communication path.
To eliminate the problem as described above, an ultra wideband communications technique using a baseband signal for an ultra wideband is commonly used. The baseband signal represents a signal having information such as audio, video or digital data. With this technique, communication is made via a communication path having strong attenuation, such as a radio-frequency transmission path and an asymmetric digital subscriber line known as ADSL.
In the ultra wideband communications technique, data is spread to a 1 GHz frequency bandwidth, for example, to transmit and receive the data. A system including the ultra wideband communications technique can produce a considerably large spectrum over a wide bandwidth, which is generally over several hundred KHz through several MHz. Therefore, the density of the spectrum becomes low, and the communication is hardly affected from other communications performed in the same frequency band at the same time. Additionally, even when the communications collide with other communications, a strong error correction code, added to the data, is used to correct the error.
By transmitting a spectrum having low density spread over a wide frequency bandwidth, the ultra wideband communications technique has the following advantages:    (1) Allows a plurality of parallel communications simultaneously in a same frequency band;    (2) Provides less or no adverse effect to other channels;    (3) Hardly affected by noise or interference from other nodes; and    (4) Has high privacy protection since the transmission is hardly traced or detected.
One of the popular techniques of ultra wideband communications is direct sequence spread spectrum (hereinafter, referred to as “DS-SS”). In the DS-SS, a data signal is multiplied by a pseudo random noise code having a fixed bandwidth so that a frequency bandwidth of a stream of input signal data is spread, modulated and transmitted.
Namely, a transmitter using the DS-SS modulates a digital signal (i.e., input data) having a narrow bandwidth and high power density to a signal having a wide bandwidth and low power density by adding a spread spectrum code with a large bit rate. A receiver detects a signal having a same stream of the input signal data as the pseudo noise code among received waves, and then reproduces the original data.
The ultra wideband communication has another communications technique using an impulse, which can be produced with a simple circuit as described in a reference of Japanese Patent Application Publication No. Kou-Hyo 11-504480, for example. Since the impulse is a flash signal and has an extremely wide frequency spectrum, negative influence to other frequency bands may effectively be reduced.
However, according to Japanese Patent Application Publication No. Kou-Hyo 11-504480, since the impulse produces signals having large power at the moment the impulse is emitted, negative influence is given to other signals.